unauthorzed billing

I sold my car in May and was going to do a change of ownership for the tracking device/account. Due to incompetence of Netstar, they couldn't figure that out. So they kept billing me every month. I was hoping for the best and being generous. When I finally had enough in September (!) and cancelled, all of a sudden they were on the "change of ownership" topic again, which at this point noone is interested in anymore. Anyway, they kept billing me and every time I sent an email reminding them of my cancellation, there seemed to be no trace of any communication, I was asked for my account number and other details again like they don't use computers and databases on their side. It got tiring. So I had the debit orders stopped. Then the debt collectors called/emailed about amounts that made no sense. I asked for detailed invoices to understand what I'm being invoiced for, I never got to speak to an actual person but it was all standard bot messages and completely useless. I follow up and remind them of/ send them the cancellation again. My next of kin gets bothered by phone calls. I try to log into my old account on their website but it (surprisingly) doesn't exist. Yet two days ago I get billed again, this time it's an amount even higher. I have literally not seen my car since May yet Netstar is happy to take my money without authorization and the inability to communicate in a professional manner via email. I prefer dealing with home affairs.